Camera systems, as understood here, comprise: an objective; an objective holder with a reception opening into which the objective is inserted; an image sensor—also referred to as camera chip—, wherein image sensor and objective are arranged relative to one another, or may be arranged relative to one another, in such a way that an image is imageable on the image sensor in focus; further, a printed circuit board—also referred to as circuit board or PCB—, to which the image sensor is contacted in an electrically conductive manner such that the electrical signals which are produced by incident light in the image sensor are forwarded on to the printed circuit board. The electrical signals may then be processed by evaluation electronics which are situated on the printed circuit board or which constitute a unit electrically connected thereto or a unit which is connected by means of other signal transferring devices. Further, the term objective is used here within the meaning of this being understood as one or more objective lenses, or else a hollow body which, as a rule, is embodied as a tube in which objective lenses are arranged. Here, focusing may be carried out by repositioning the lenses or by twisting or displacing the tube, with the lenses situated therein, in the objective holder. The following may be used as a camera or camera chip: a CMOS, a CCD, a range imaging camera (depth measurement with time of flight), a photodiode array, focal plane arrays, a thermocam, multispectral sensors. Depending on requirements, spectral filters may also be provided in the optical path of the camera system, i.e., for example, attached in the tube or on the camera chip. The camera system may be designed to image wavelengths in the visible range, but also in the near and far infrared wavelength range.
FIG. 1 shows a typical example of a camera system 10 according to the prior art. In the example shown here, the objective 12 of the camera system 10 is provided with a male thread 14, with the aid of which the objective 12 is screwed into a female thread of a reception opening 17 of an objective holder 16. The reception opening 17 is continuous and forms a recess 25 in the extension of the optical axis 13 of the objective 12, the image sensor 22 being arranged in said recess in the assembled state. Here, the sensor plane of the image sensor 22 is arranged as orthogonal as possible to the optical axis 13 of the objective 12 and the center of the sensor plane is as flush as possible with the optical axis 13.
The image sensor 22 is fixed directly to a printed circuit board 20, with the fixation 24 e.g. being realized by a soldered connection or a welded connection such that the image sensor 22 is connected to the printed circuit board 20 in an electrically conductive manner and the electrical signals which are produced by incident light in the image sensor 22 can be transmitted to the printed circuit board for processing purposes. Soldered connections in the form of ball grid arrays were found to be particularly easy to produce and found to be a particularly stable, conductive, direct connection 24 between image sensor 22 and printed circuit board 20.
During the assembly, the printed circuit board 20 with the image sensor 22 affixed thereon is fastened to the objective holder 16 on that side of the objective holder 16 that lies opposite to the objective 12 in such a way that, firstly, the continuous opening 17 in the objective holder 16 is sealed by the printed circuit board 20, as a result of which the recess 25 emerges, and, secondly, the image sensor 22 is then placed into the recess 25, as described above, as perpendicularly and as centrally as possible in relation to the optical axis 13. To this end, the objective holder 16 has threaded bores 18 for receiving screws 31. The printed circuit board 20 has continuous attachment openings 21, which are arranged in such a way that they lie flush with the threaded bores 18 of the objective holder 16 when the image sensor 22 that is affixed to the printed circuit board 20 is aligned approximately perpendicular to the optical axis 13 with the sensor plane thereof and approximately flush with the optical axis 13 of the objective 16 with the center thereof. Then, the printed circuit board 20 is connected, in a detachable but secure manner, to the objective holder 16 in this position with the aid of the screws 31, which are passed through the attachment openings 21 of the printed circuit board 20 and twisted into the threaded bores 18. As may be identified from FIG. 1, the objective 12 and image sensor 22 are adjustable relative to one another in the lateral position—at least in respect of a plane that is aligned approximately perpendicular to the optical axis 13 of the objective—if the screws 31 have not been tightened completely and said objective and image sensor may then be affixed in this adjusted position by tightening the screws. Alternatively, a defined lateral position prescription may be realized in place of the lateral adjustability, for example by way of mechanical positioning means, such as e.g. positioning pins arranged on the objective holder, said mechanical positioning means engaging into positioning recesses of the printed circuit board that are embodied in a mirror inverted manner, or vice versa. Additional attachment may also be ensured e.g. by adhesive or lacquer points.
In the example shown here, the distance between the objective 12 and image sensor 22 may be adjusted for focusing purposes, for example by virtue of the objective, with the objective thread 14 thereof, being screwed deeper into the reception opening 17 of the objective holder 16 or being rotated further out of said reception opening. In this way, the image sensor 22 and the objective 12 may be arranged relative to one another along the optical axis in such a way that an image projected onto the image sensor 22 via the objective 12 is imaged in focus on the image sensor 22.
Camera systems of the type described above are often constituents of relatively high-quality photo cameras and then, as a rule, also have a variable focus. In a simple embodiment of such a camera system, a focusing device ensures focus. Here, the objective thread is actuated in a motor-driven manner, rendering it possible to variably set the distance between the image sensor and the objective correspondingly as described above by twisting the objective in the objective holder, such that a sharp image can be produced on the image sensor over a broad range. The demands on camera systems, which are used in such photo cameras, in respect of the ability to calibrate or in respect of accuracies or tolerable aberrations which, for example, may also emerge from tremors or temperature variations, may, as a rule, be satisfied well using the currently available structures and technical components. However, if this relates to highest precision of image capture, such focusing devices often act as sources of errors on account of their many parts that are movable in relation to one another.
Thus, the predetermined demands are often not satisfied by the above-mentioned camera systems when they are constituents of precise measuring appliances. Aberrations which, especially quantitatively, are often predictable with difficulties, or not at all, are often observed with such camera systems, even in measuring appliances in which a focusing device—and hence one of the sources of errors (see above)—can be dispensed with in the camera system (e.g. because the distance variation which needs to be capturable by the camera system is small). In addition to suddenly occurring aberrations, e.g. after tremors, shocks or sudden temperature variations, these also include, inter alia, measurement value drift and/or hysteresis effects. Due to the lack of predictability, a compensation of such aberrations by means of software, or else an appropriate calibration, is often only possible with great difficulties, or not at all, or a calibration must be carried out very frequently.
Even if—as described above—a focusing device is not provided, the connection between objective and objective holder nevertheless remains as a source of error in many cases since, as explained above, the connection between the objective and the image sensor on the printed circuit board is generally embodied in the form of a screw connection that is realized by the objective holder.
A further source of error in precision applications can lie in different coefficients of thermal expansion within the objective, i.e. the objective lenses on the one hand and the tube carrying the lenses on the other hand, but also between the objective and objective holder. As a rule, the objective lenses are manufactured from glass and therefore have a comparatively small coefficient of thermal expansion aG1 in the region of aG1=0.5*10−6K−1 to 9*10−6K−1. By contrast, the tube or objective holder are often manufactured from aluminum or an aluminum alloy, and so the coefficient of thermal expansion aOb thereof lies in the region of aOb=22*10−6K−1 to 24*10−6K−1.
The connection or attachment of the image sensor on the printed circuit board may also constitute a further source of error in respect of precise measurements using the camera system. As already mentioned above, very different options are known for this attachment, such as soldering and welding and an embodiment of the image sensor as an SMD chip with connection pins for THT connection technology. The option for direct connection between image sensor and printed circuit board which, of the known connections, is probably the most mechanically rigid is the connection by means of a ball grid array (BGA). In the ball grid array, solder beads arranged in a grid are melted by reflow soldering, i.e. by heating in a soldering furnace, such that, subsequently, the molten and re-solidified soldering material securely connects contact pads of the image sensor with contact pads of the printed circuit board.
A further source of error arising when such camera systems are used in measuring appliances for highly precise measurements is also formed by the conventionally used printed circuit boards, which, as a rule, are manufactured from FR4 with copper conductor tracks. FR4 is a composite material made of a low-flammable and flame-retardant epoxy resin and glass fiber tissue, which has anisotropic, i.e. direction-dependent, coefficients of thermal expansion of typically ax=14*10−6K−1; ay=18*10−6K−1; az=60*10−6K−1 (the indices x, y, z represent the Cartesian directions). Slots for pin-established camera chips or for positioning aids for SMD camera chips that can be soldered on directly and also the continuous attachment openings, by means of which the printed circuit board—usually with the aid of screws—is connected to the objective holder and the objective situated therein, cannot be manufactured in the printed circuit board with an accuracy that is sufficient for the demands of such measuring appliances. That is why the position of the camera chip on the printed circuit board, the position of the printed circuit board in relation to the objective holder and hence also the position of the camera chip in relation to the objective cannot be set with sufficient accuracy or the positions thereof change in relation to one another, e.g. in the case of tremors or temperature variations, etc.
CN202004864U therefore proposes a positioning aid in the form of an aluminum template, which remains in the camera system, even after the positioning. The aluminum template has a precisely defined slot for the camera chip and it is welded onto the printed circuit board together with the camera chip. The aluminum template is only slightly larger than the camera chip and has passage openings to the side of the camera chip, said passage openings being flush with the attachment openings of the printed circuit board in the welded-on state. Since, according to assertions made in this document, the aluminum template with the slot thereof for the camera chip and with the passage openings thereof can be worked on much more precisely than the printed circuit board, substantially more accurate positioning of the individual components in relation to one another may be achieved during the assembly with the aid of the aluminum template.
In other known camera systems, the errors which arise, in particular, from manufacturing tolerances of the attachment openings of the printed circuit board are to be avoided by a glass-based connection structure. Here, a rear side of the image sensor is connected to a printed circuit board in a conventional manner; by contrast, the front face of the image sensor is adhesively bonded to a carrier glass. The carrier glass projects beyond the edge of the image sensor. This projecting edge of the carrier glass is securely connected to the objective holder, into which the objective is screwed, with the side thereof facing away from the image sensor, for example by means of a temperature-stable adhesive bond. In this way, the printed circuit board is only connected to the objective holder via the image sensor. All sources of error which may be due to connecting the image sensor via a printed circuit board and screws to the objective holder have been removed by this structure. By contrast, it appears to be comparatively easily possible to introduce positioning aids for the attachment of the image sensor on the one hand and for the attachment of the glass plate on the objective holder on the other hand into the glass plate with a high precision. However, this structure is very complicated in terms of the production thereof and the printed circuit board is only connected to the camera system by means of the conventional joints to the image sensor such that, in the case of tremors or shocks, the stability of the image sensor-printed circuit board connection must be ensured by the joints only.